I LOVED this story and wish very much that there was more, as your "brothers" Arthur and Merlin are a beautiful description of a side of the pair that we only saw a few times throughout the entire show. We NEED more "not-quite-but-more-or-less brothers" Arthur and Merlin stories!
Also, the Kingdom of Caerleon WAS mentioned in an episode, I believe it was 4x05, but anyway, it was ruled by Queen Annis and when Arthur kills her husband at Agravaine's urging, Caerleon and Camelot almost go to war. Luckily Arthur comes to his senses and proves, with Merlin's help, just what a GOOD King he is/will be.